devel/llvm10: un-deprecate, shrink default build

Remove expiration and deprecation as Arm platforms will require llvm10
as long as we support lang/ghc810.

Reduce the default builds to just the native backend(s), lld, and clang.
This is more than lang/ghc810 needs (it just needs LLVM), but also
covers lang/silq until its dependency on lld is removed.